JP2005110014A - Portable terminal, communication system and communication method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a portable terminal capable of preventing a line throughput from lowering while preventing communication quality from being degraded caused by the timing difference to a base station at the time of performing a soft handover. <P>SOLUTION: This portable terminal 10 for making a communication with a plurality of base stations 30 and 40 by using an OFDM (Orthogonal Frequency Division Multiplexing) signal is provided with: communication means 114 and 200 for transmitting and receiving the OFDM signal to/from the base stations; a handover detecting means 222 for detecting start timing of handover; and OFDM signal control means 120 and 220 for changing the guard time length of the OFDM signal when the soft handover detecting means 222 detects the start timing of the handover. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a portable terminal that communicates with a base station using an OFDM (Orthogonal Frequency Division Multiplexing) signal, a communication system including the portable terminal, and a communication method.

  In the conventional OFDM signal communication system, a guard time is included in the OFDM signal in order to suppress degradation of the OFDM signal due to the influence of the delay wave. In this case, a guard time having a predetermined length is inserted. A technique for determining the length of a guard time to be inserted based on a delay time due to multipath is known (for example, Patent Document 1).

JP 2002-374223 A

  On the other hand, in communication using an OFDM signal between a mobile terminal and a base station, handover occurs as the mobile terminal moves. At the time of this handover, it is known that the OFDM signal deteriorates due to a timing shift between the OFDM signals transmitted from the two base stations.

  However, in the conventional OFDM signal communication system, a technique for dealing with temporary degradation of the OFDM signal at the time of handover has not been developed. Further, when the change of the guard time length is made dependent on the change of the propagation environment, there is a problem that the required specification of the transmission timing synchronization for the base station becomes strict. In addition, in the communication system that performs spread spectrum modulation, there is a problem that the line throughput decreases when the length of the guard time is changed.

  An object of the present invention is to provide a communication method capable of preventing a reduction in line throughput while preventing a deterioration in communication quality due to a timing difference with respect to a base station in an OFDM signal communication system. To do.

  In order to solve the above-described problems and achieve the object, the present invention is a portable terminal that communicates with a plurality of base stations using OFDM signals, and communicates the base station with the OFDM signals. Means, a handover detecting means for detecting a start timing of the handover, and an OFDM signal control means for changing a guard time length of the OFDM signal when the handover detecting means detects the start timing of the handover. It is characterized by that.

  Since the mobile terminal according to the present invention makes the guard time longer than usual during handover, it is possible to prevent deterioration in communication quality by reducing synchronization shift between base stations due to propagation delay fluctuation and the like. There is an effect.

  Embodiments of a mobile terminal, a communication system, and a communication method according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

  FIG. 1 illustrates an overall configuration of a communication system 1 including a mobile terminal 10 according to the first embodiment. The communication system 1 includes a mobile terminal 10, a first base station 30, a second base station 40, and a switching center 50. The mobile terminal 10 and the base station of this system communicate with each other using a modulation scheme based on OFDM, and a cellular system is assumed in which a plurality of base stations expand the cover area. The mobile terminal 10 communicates with the first base station 30 and the second base station 40 as it moves. The switching center 50 controls handover.

  In the embodiment, the case where the mobile terminal 10 is handed over from the first base station 30 to the second base station 40 will be described. However, the number of base stations that can be handed over in the communication system 1 is limited to two. It is not a thing.

  FIG. 2 is a diagram illustrating a configuration of the transmission processing unit 11 that performs transmission processing in the mobile terminal 10 according to the first embodiment. The transmission processing unit 11 includes a channel encoder 100, a modulator 102, an OFDM frame generation unit 104, an inverse fast Fourier transform (IFFT) unit 106, a guard interval addition unit 108, a D / A conversion unit 110, a radio A frequency modulation unit (RF) 112, a transmission unit 114, and a control unit 120 are included.

  The channel encoder 100 encodes data to be transmitted to the base station. The modulator 102 performs OFDM modulation on the encoded data. The OFDM frame generation unit 104 generates an OFDM frame including a plurality of OFDM symbols.

  IFFT section 106 performs inverse fast Fourier transform on the OFDM frame generated by OFDM frame generation section 104. The guard interval adding unit 108 adds a guard interval to the information subjected to the fast Fourier inverse transform by the IFFT unit 106.

  Further, the control unit 120 controls the guard interval adding unit 108. Specifically, the length of the guard time to be added by the guard interval adding unit 108 is changed based on whether or not the transition is made to the handover. More specifically, the guard time length is set in advance for each of the handover time when the handover is performed and the normal time when the handover is not performed, and the guard time length is switched. The control unit 120 according to the present embodiment constitutes an OFDM signal control unit of the present invention.

  Then, the OFDM signal is modulated by the D / A converter 110 and the radio frequency modulator (RF) 112 and transmitted to the outside via the transmitter 114.

  FIG. 3 is a diagram illustrating a configuration of the reception processing unit 12 that performs reception processing in the mobile terminal 10. The reception processing unit 12 includes a reception unit 200, an RF unit 202, an A / D conversion unit 204, a guard interval removal unit 206, a fast Fourier transform (FFT) unit 208, an OFDM frame separation unit 210, and a demodulator. 212, a channel decoder 214, a control unit 220, and a handover determination unit 222.

  The receiving unit 200 receives an OFDM signal. The OFDM signal is down-converted and analog-digital converted by the RF unit 202 and the A / D conversion unit 204 and output to the guard interval removal unit 206. The guard interval removal unit 206 removes the guard interval included in the OFDM signal. In addition, the control unit 220 controls the guard interval removal unit 206. Specifically, the control unit 220 changes the length of the guard time to be removed by the guard interval removal unit 206 based on whether or not the handover is performed. The guard interval removal unit 206 constitutes the OFDM signal control means of the present invention. The handover determination unit 222 regularly observes the propagation environment. Specifically, the radio field intensity from each base station is measured.

  The FFT unit 208 performs fast Fourier transform on the information from which the guard interval has been removed. The OFDM frame separation unit 210 separates data of one OFDM symbol. The demodulator 212 performs OFDM demodulation on the data received from the OFDM frame separation unit 210. The channel decoder 214 further decodes the demodulated data.

  4A and 4B schematically illustrate OFDM symbols transmitted and received between the mobile terminal 10 and the base stations 30 and 40. FIG. In the communication system 1 according to the present embodiment, OFDM symbols having different guard time lengths are set during handover and during normal operation.

  FIG. 4A illustrates an OFDM symbol transmitted / received in a normal time. The guard time length in a symbol is set to the expected multipath propagation delay length. FIG. 4B illustrates an OFDM symbol transmitted / received at the time of handover. Thus, the OFDM symbol transmitted / received at the time of handover has a longer guard time than the OFDM symbol transmitted / received at the normal time. By doing so, it is possible to increase resistance to a reception timing difference between the first base station 30 and the second base station 40.

  In this embodiment, the OFDM symbol length transmitted / received during handover is twice as long as the OFDM symbol length transmitted / received during normal operation. The length of the data portion in the OFDM symbol is constant regardless of whether soft handover is performed. That is, the symbol length of the OFDM symbol transmitted / received at the time of handover is longer than the symbol length of the OFDM symbol transmitted / received at the normal time by the length of the guard time.

  In this embodiment, at the time of handover, the OFDM symbol length is changed to twice the normal length, but as another example, the length is changed to an integral multiple such as 4 times or 8 times. Also good.

  FIG. 5 schematically illustrates how soft handover is implemented in the OFDM signal communication system 1 according to the first embodiment. In the communication system 1 according to the present embodiment, a communication terminal and a base station communicate with each other using a modulation scheme based on OFDM. And it is a system of the cellular form which a some base station expand | deploys a cover area in a plane.

  In FIG. 5, the first base station 30 communicates with communication devices in the first region 300. In addition, the second base station 40 communicates with communication devices in the second area 400.

  Here, the handover when the mobile terminal 10 moves from the position 310 in the first area 300 to the position 410 in the second area 400 will be described.

  When the mobile terminal 10 is at the position 310, the distance between the mobile terminal 10 and the first base station 30 is short, and sufficient wireless communication line quality can be maintained. However, when the mobile terminal 10 moves and is placed at the position 320 close to the boundary of the first region, the average communication quality is lower than when it is placed at the position 310. Furthermore, at this time, the radio wave from the second base station 40 is strongly received.

  An OFDM signal has a property that it can be operated at the same frequency (single frequency network: SFN) as long as it includes radio waves containing the same information. Therefore, even when the mobile terminal 10 is placed at the position 320, it is possible to increase the received signal power by receiving radio waves from both the first base station 30 and the second base station 40, Thereby, soft handover can be realized.

  However, in this case, if the timing difference when the mobile terminal 10 receives radio waves from both base stations does not fall within the guard time of the OFDM symbol, inter-symbol interference occurs and the reception quality deteriorates. For this reason, at the time of handover, a sufficiently long guard time is required to prevent intersymbol interference.

  On the other hand, the mobile terminal 10 according to the present embodiment can perform communication using an OFDM symbol including a guard time having a sufficient length to prevent intersymbol interference at the time of handover. In addition, since the guard time length is returned to the original length at times other than handover, that is, communication is performed using OFDM symbols including a guard time shorter than that at the time of handover, a decrease in throughput is minimized. be able to.

  FIG. 6 is a flowchart showing processing of the communication system 1 when handover is performed. Processing when the mobile terminal 10 switches the communication partner from the first base station 30 to the second base station 40 will be described with reference to FIG.

  The mobile terminal 10 regularly observes the propagation environment (step S100). Then, when the intensity of the radio wave from the second base station becomes equal to or higher than the reference value, a handover request indicating that it can move to the handover state is transmitted to the first base station 30 and the switching center 50 (step S102). At this time, the first base station 30 inquires of the switching center 50 at the same time whether or not radio resources in the second base station 40 that is the handover destination are available. Here, the reference value is a preset value.

  When receiving the handover request and the inquiry about the radio resource, the switching center 50 inquires of the second base station 40 whether or not the radio resource can be used (step S104). If there is a free radio resource in the second base station 40, a resource permission indicating that the radio resource is available is received from the second base station 40 (step S106). Next, the switching center 50 transmits a handover permission indicating permission of handover transition to the first base station 30 and the mobile terminal 10 (step S110). Furthermore, the switching center 50 performs setting for opening the line from the second base station 40 (step S112).

  Thereby, the switching center 50 sets the guard time of the OFDM symbol at the time of handover. Similarly, the second base station 40 sets an OFDM symbol guard time during handover (step S120).

  Moreover, the 1st base station 30 and the portable terminal 10 will change guard time length, if handover permission is received (step S122, step S124). In the present embodiment, the guard time length is made longer than the normal length, and the symbol length is doubled. As described above, in the switching center 50, the first base station 30, the second base station 40, and the mobile terminal 10, OFDM symbols including the same length of guard time are set at the same timing.

  FIG. 7 is a flowchart showing processing of the communication system 1 when the handover is completed. When the handover determining unit 122 of the mobile terminal 10 determines that the handoff is completed (step S100), a handoff completion notification indicating that the handoff is completed is transmitted to the first base station 30 and the second base station 40 (step S130). And the control part 120 returns guard time length to the original length (step S132). Here, the original length is a length that is normally set as a guard time length.

  Further, when receiving the handoff end notification from the mobile terminal 10, the first base station 30 and the second base station 40 each shorten the guard time length and change the symbol length to ½ times that during handover. That is, the normal length is restored. As described above, in the second base station 40, the first base station 30, and the mobile terminal 10, OFDM symbols including the same length of guard time are set at the same timing.

  Thus, after the handoff is completed, the overhead in the system can be reduced and the communication efficiency can be improved by reducing the length of the guard time.

  Next, the communication system 1 according to the second embodiment will be described. In the communication system 1 according to the second embodiment, the length of the guard time is determined based on the timing difference between radio waves at the first base station 30 and the second base station 40 at the time of handover.

  FIG. 8 is a flowchart of a process when transitioning to a handover in the communication system 1 according to the second embodiment. In the second embodiment, the switching center 50 transmits a resource request to the second base station 40 (step S104), inquires the timing of the radio wave to the second base station 40 (step S105), and responds to the inquiry. As a response, a radio wave timing report is received (step S107).

  The switching center 50 determines the guard time length based on the received timing report (step S108). The length of the guard time determined here is a value longer than the length of the guard time set when handover is not performed. Preferably, the length of the guard time is increased so that the symbol length is twice the symbol length set when handover is not performed.

  And the determined guard time length is notified to the 2nd base station 40, the 1st base station 30, and the portable terminal 10 (step S114, step S115). And the 2nd base station 40, the 1st base station 30, and the portable terminal 10 set the guard time of the same length at the same timing.

  FIG. 9 shows a guard time length table 500 used when the switching center 50 determines the guard time. The guard time length table 500 associates a timing difference between two base stations with a guard time length. When the switching center 50 identifies the timing difference between the two base stations, the switching center 50 determines the guard time length associated with the identified timing difference in the guard time length table 500.

  Thus, since the timing difference and the guard time length are associated with each other in the guard time length table 500, the switching center 50 can set an appropriate guard time length based on the timing difference.

  Here, a process for determining the guard time will be described. For example, a numerical example with respect to the guard time when IEEE 802.11a is used as a reference will be shown. The guard time must be determined based on the propagation delay time expected in the system. In the case of soft handover, the guard time is determined based on the distance difference between the terminal and each base station + the base station time accuracy.

  For example, when the cell radius is several tens of meters and the OFDM symbol length is 4 μs, the guard time is preferably 0.8 μs. Here, a guard time of 0.8 μs corresponds to a propagation delay difference of 240 m.

  Assuming that the data rate in the communication system 1 is the same as in IEEE 802.11a and the cell radius is on the order of several hundreds of meters, a guard time of about 1 OFDM symbol is required to absorb the difference in distance between the terminal and the base station. Further, in the case of the order of several kilometers, it is necessary to consider the time difference of the guard time corresponding to several symbols. On the other hand, when shortening the OFDM symbol length in order to increase the transmission rate, it is necessary to relatively lengthen the guard time.

  Other configurations and processes of the communication system 1 are the same as those of the communication system 1 described in the first embodiment.

  Next, the communication system 1 according to the third embodiment will be described. In the communication system 1 according to the third embodiment, the mobile terminal 10 measures a timing difference between two base stations related to a handover. Based on the timing difference, the switching center 50 determines the guard time length during handover. In this respect, the communication system 1 according to the third embodiment is different from the communication system 1 according to another embodiment.

  FIG. 10 is a block diagram of functional configurations of the reception processing unit 12 and the transmission process of the mobile terminal 10 according to the third embodiment.

  In the third embodiment, the reference data is transmitted to the first base station 30 and the second base station 40 via the transmission unit 232 and the D / A conversion unit 234. When receiving the reference data, the first base station 30 and the second base station 40 return the reference data to the mobile terminal 10 by return. Then, the timing difference measuring unit 230 measures the timing difference between the reference signals received from the first base station 30 and the second base station 40 as a reply to the reference data.

  The control unit 220 sends a timing difference report indicating the timing difference measured by the timing difference measurement unit 230 to the transmission unit 232. The timing difference report is transmitted to the switching center 50 via the transmission unit 232, the D / A conversion unit 234, the RF unit 202, and the reception unit 200. Here, the transmission unit 232 has the functions of the channel encoder 100, the modulator 102, the OFDM frame generation unit 104, the IFFT unit 106, and the guard interval addition unit 108 described in the first embodiment with reference to FIG. Yes.

  The control unit 220 also controls the guard interval removal unit 206 in the same manner as the control unit 220 according to the first embodiment.

  The switching center 50 according to the third embodiment includes a guard time length table 500 as in the switching center 50 according to the second embodiment. Then, the guard time length is determined based on the timing difference indicated in the timing difference report received from the mobile terminal 10.

  FIG. 11 is a flowchart of a process when the mobile terminal 10 according to the third embodiment shifts to a handover. In the third embodiment, the mobile terminal 10 regularly receives reference signals from the first base station 30 and the second base station 40 (step S140). Then, the timing difference measurement unit 230 measures the timing difference based on each timing at which the reference signal is received from each of the first base station 30 and the second base station 40 (step S142). Next, the mobile terminal 10 transmits a handover request (step S102), and transmits a timing difference report indicating the measured timing difference to the switching center 50 (step S103).

  The switching center 50 determines a guard time based on the timing difference report (step S108), and notifies the determined guard time to the second base station 40, the first base station 30, and the mobile terminal 10 (step S114, step). S115).

  Other configurations and operations of the communication system 1 according to the third embodiment are the same as the configurations and operations in the communication system 1 according to the first embodiment.

  In the present embodiment, the timing difference measurement unit 230 of the reception processing unit 12 measures the timing difference based on the signal output from the A / D conversion unit 204, but as another example, the timing difference measurement The unit 230 may measure the timing difference based on the signal output from the guard interval removal unit 206.

  Other configurations and processes of the communication system 1 according to the third embodiment are the same as those of the communication system 1 described in the second embodiment.

  Next, the communication system 1 according to the fourth embodiment will be described. In the communication system 1 according to the fourth embodiment, as in the communication system 1 according to the third embodiment, the timing difference measurement unit 230 of the mobile terminal 10 measures the timing difference between the first base station 30 and the second base station 40. . In the communication system 1 according to the fourth embodiment, the control unit 220 of the mobile terminal 10 further determines the length of the guard time based on the timing difference.

  The control unit 220 includes the guard time length table 500 described in the second embodiment, and determines the guard time length using the guard time length table 500. Moreover, the control part 220 comprises the guard time length determination means of this invention. The communication system 1 according to the fourth embodiment is different from the communication system 1 according to another embodiment in this respect.

  FIG. 12 is a flowchart of a process when transitioning to a handover in the communication system 1 according to the fourth embodiment. In the communication system 1 according to the fourth embodiment, when the mobile terminal 10 receives the reference signal from the first base station 30 and the second base station 40 (step S140), the timing difference measurement unit 230 measures the timing difference (step S142). ).

  Next, the control unit 220 determines the guard time length associated with the timing difference measured by the timing difference measurement unit 230 in the guard time length table 500 as the guard time length at the time of handover (step S144). And the guard time length which shows the guard time length determined by the control part 220 is transmitted to the switching center 50, and it reports (step S146).

  And the switching center 50 notifies the reported guard time length to the 1st base station 30, the 2nd base station 40, and the portable terminal 10 (step S114, 115). In the present embodiment, since the mobile terminal 10 recognizes the guard time length, the switching center 50 may not notify the mobile terminal 10 of the guard time length. Through the above processing, the first base station 30, the second base station 40, and the mobile terminal 10 can set the same guard time length during handover.

  Other configurations and operations of the communication system 1 according to the fourth embodiment are the same as those in the communication system 1 according to the third embodiment.

  Next, the communication system 1 according to the fifth embodiment will be described. In the communication system 1 according to the fifth embodiment, the mobile terminal 10 performs OFDM modulation on the transmission signal and directly performs spread spectrum. In the communication system 1 according to the fifth embodiment, the length of the guard time and the spreading factor are changed between when the handover is performed and when the handover is not performed. Specifically, the spreading factor at the time of handover and the spreading factor at the normal time are set in advance, and the spreading factor is changed based on the presence or absence of handover. In this respect, the communication system 1 according to the fifth embodiment is different from the communication system 1 according to another embodiment.

  FIG. 13 is a block diagram of a functional configuration of the transmission processing unit 11 of the mobile terminal 10 according to the fifth embodiment. In addition to the functional configuration of the transmission processing unit 11 according to the first embodiment, the transmission processing unit 11 according to the fifth embodiment further includes a spreader 140 that performs direct spectrum spreading. Further, the control unit 120 controls the guard interval adding unit 108 in the same manner as the control unit 120 according to the first embodiment. The control unit 120 according to the fifth embodiment further controls the diffuser 140. Specifically, the spreading factor in the spread spectrum performed by the spreader 140 is changed based on whether or not the transition to the handover is performed.

  FIG. 14 is a block diagram of a functional configuration of the reception processing unit 12 of the mobile terminal 10 according to the fifth embodiment. In addition to the functional configuration of the reception processing unit 12 according to the first embodiment, the reception processing unit 12 according to the fifth embodiment further includes a despreader 240 that performs despreading. In addition, the control unit 220 according to the fifth embodiment controls the guard interval removal unit 206 in the same manner as the control unit 220 according to the first embodiment. The control unit 120 according to the fifth embodiment further controls the despreader 240. Specifically, the spreading factor in the despreading performed by the despreader 240 is changed based on whether or not the transition to the handover is performed.

  FIGS. 15A and 15B schematically illustrate OFDM symbols transmitted and received between the mobile terminal 10 and the base stations 30 and 40 in the communication system 1 according to the fifth embodiment.

  FIG. 15A illustrates an OFDM symbol transmitted / received at normal time. FIG. 15-2 illustrates OFDM symbols transmitted and received at the time of handover.

  Normally, OFDM symbols are spread at a spreading factor of 4. Further, the guard time in the OFDM symbol is set longer than the expected multipath propagation delay. In the OFDM symbol at the time of handover, the guard time is set longer than the normal guard time length. Thereby, the tolerance with respect to the difference of the reception timing from the 1st base station 30 and the 2nd base station 40 can be improved.

  Furthermore, the spreading factor is set low as the guard time is set longer. The OFDM symbol shown in FIG. 15-2 is spread with a spreading factor of 2. In this way, by reducing the spreading factor, it is possible to ensure continuous throughput and continue communication without lowering the data rate.

  FIG. 16 is a flowchart of a process when transitioning to a handover in the communication system 1 according to the fifth embodiment. In the communication system 1 according to the fifth embodiment, when a handover permission is transmitted from the switching center 50 (step S110) and a process of opening a line to the second base station 40 is performed (step S112), the second base station In the transmission setting stage 40 (step S120), the spreading factor at the time of handover, that is, 2 is set.

  In addition, the first base station 30 and the mobile terminal 10 both change the guard time length (step S122, step Sq124) and change the spreading factor (step S123, step S125). As described above, in the communication system 1 according to the fifth embodiment, at the time of handover, each device simultaneously changes the guard time length and the spreading factor to the same value.

  FIG. 17 is a flowchart of a process when the communication system 1 according to the fifth embodiment shifts from a handover to a normal state. In the communication system 1 according to the fifth embodiment, when the mobile terminal 10 determines that the handoff is completed (Step S100), the guard time length is returned to the original length (Step S132), and the spreading factor is set to the original rate. Return (step S152). Here, the original spreading factor is a spreading factor that should be set in a normal state, and is 4 in this embodiment.

  Further, when receiving the handoff end notification from the mobile terminal 10, the first base station 30 and the second base station 40 each return the guard time length to the normal length (steps S134 and S136), and the spreading factor is set to normal. (Step S154, step S156). As described above, the OFDM symbols including the guard time of the same length and spread at the same spreading factor in the second base station 40, the first base station 30, and the mobile terminal 10 are set.

  Thus, at the end of the handoff, the overhead in the system can be reduced and the communication efficiency can be improved by shortening the guard time length and increasing the spreading factor.

  Other configurations and operations of the communication system 1 according to the fifth embodiment are the same as the configurations and operations in the communication system 1 according to the first embodiment.

  Next, the communication system 1 according to the sixth embodiment will be described. In the communication system 1 according to the sixth embodiment, a decrease in throughput caused by changing the guard time during soft handover is compensated by changing other parameters. In this respect, the communication system 1 according to the sixth embodiment is different from the communication system 1 according to another embodiment.

  FIG. 18 is a block diagram of a functional configuration of the transmission processing unit 11 of the mobile terminal 10 according to the sixth embodiment. The reception processing unit 12 according to the sixth embodiment has the same functional configuration as that of the reception processing unit 12 according to the fifth embodiment. In the reception processing unit 12 according to the sixth embodiment, the control unit 120 controls processing in the channel encoder 100. The control unit 120 also controls the diffuser 140 and the guard interval adding unit 108 in the same manner as the control unit 120 according to the fifth embodiment.

  FIG. 19 is a block diagram of a functional configuration of the transmission processing unit 11 of the mobile terminal 10 according to the sixth embodiment. The reception processing unit 12 according to the sixth embodiment has the same functional configuration as that of the reception processing unit 12 according to the fifth embodiment. In the reception processing unit 12 according to the sixth embodiment, the control unit 220 controls processing in the channel decoder 214. The control unit 220 also controls the guard interval removal unit 206 and the despreader 212 in the same manner as the control unit 220 according to the fifth embodiment.

  Then, the channel encoder 100 and the channel decoder 214 change the guard time based on the instructions from the control unit 120 and the control unit 220 using the error correction coding rate and parameters for puncturing and repetition, respectively. To compensate for the decrease in throughput.

  These parameters also affect the quality of the radio line. Therefore, the change values of these parameters need to be determined by the trade-off between throughput and quality. By using such a method, it is possible to increase the tolerance against the reception timing difference between the first base station 30 and the second base station 40 at the time of soft handover, and the coding rate and puncturing are increased by the length of the guard time. / By changing the repetition parameter, it is possible to ensure throughput and continue communication without lowering the data rate.

  Other configurations and operations of the communication system 1 according to the sixth embodiment are the same as those in the communication system 1 according to the fifth embodiment.

  As described above, the present invention has been described using the embodiment, but various changes or improvements can be added to the above embodiment.

  As described above, the portable terminal and the like according to the present invention are useful for a communication apparatus using an OFDM signal, and are particularly suitable for a portable terminal and the like.

1 is a diagram illustrating an overall configuration of a communication system 1 including a mobile terminal 10 according to a first embodiment. FIG. 3 is a diagram illustrating a configuration of a transmission processing unit 11 of the mobile terminal 10 according to the first embodiment. It is a figure which shows the structure of the reception process part 12 which performs the reception process in the portable terminal. It is a figure which shows typically the OFDM symbol transmitted / received at the time of normal. It is a figure which shows typically the OFDM symbol transmitted / received at the time of a hand-over. It is a figure which shows typically the mode of soft handover realization in the OFDM signal communication system 1 concerning Example 1. FIG. It is a flowchart which shows the process of the communication system 1 when a handover is performed. It is a flowchart which shows the process of the communication system 1 when ending a handover. 7 is a flowchart illustrating a process when transitioning to a handover in the communication system 1 according to the second embodiment. It is a figure which shows the guard time length table 500 utilized when the switching center 50 concerning Example 2 determines guard time. FIG. 10 is a block diagram illustrating a functional configuration of a transmission processing unit 11 of the mobile terminal 10 according to the third embodiment. 12 is a flowchart illustrating a process when transitioning to handover in the communication system 1 according to the third embodiment. 14 is a flowchart illustrating a process when transitioning to a handover in the communication system 1 according to the fourth embodiment. FIG. 10 is a block diagram illustrating a functional configuration of a transmission processing unit 11 of a mobile terminal 10 according to a fifth embodiment. FIG. 10 is a block diagram illustrating a functional configuration of a reception processing unit 12 of the mobile terminal 10 according to the fifth embodiment. The OFDM symbol transmitted / received at the normal time between the portable terminal 10 and the base stations 30 and 40 in the communication system 1 concerning Example 5 is shown. The OFDM symbol transmitted / received at the time of the hand-over between the portable terminal 10 and the base stations 30 and 40 in the communication system 1 concerning Example 5 is shown. FIG. 10 is a flowchart illustrating processing when shifting to handover in the communication system 1 according to the fifth embodiment. FIG. 10 is a flowchart illustrating processing when shifting from a handover to a normal state in the communication system 1 according to the fifth embodiment. FIG. 10 is a block diagram illustrating a functional configuration of a transmission processing unit 11 of a mobile terminal 10 according to a sixth embodiment. FIG. 10 is a block diagram illustrating a functional configuration of a transmission processing unit 11 of a mobile terminal 10 according to a sixth embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Communication system 10 Portable terminal 11 Transmission processing part 12 Reception processing part 30,40 Base station 50 Switching center 100 Channel encoder 102 Modulator 104 OFDM frame generation part 106 IFFT part 108 Guard interval addition part 110 D / A conversion part 112 RF part 114 Transmitter 120 Control Unit 122 Handover Determination Unit 140 Spreader 200 Reception Unit 202 RF Unit 204 A / D Conversion Unit 206 Guard Interval Removal Unit 208 FFT Unit 210 OFDM Frame Separation Unit 212 Demodulator 214 Channel Decoder 220 Control Unit 222 Handover Determination Unit 230 timing difference measurement unit 232 transmission unit 240 despreader 500 guard time length table

Claims (9)

A portable terminal that communicates with a plurality of base stations using an OFDM (Orthogonal Frequency Division Multiplexing) signal,
Communication means for transmitting and receiving the base station and the OFDM signal;
A handover detecting means for detecting a handover start timing;
An OFDM signal control means for changing a guard time length of the OFDM signal when the handover detection means detects the start timing of the handover.   The mobile terminal according to claim 1, wherein the OFDM signal control means makes the guard time length longer than the present time when the handover detection means detects the start timing of the handover.   The OFDM signal control means changes the guard time length of the OFDM signal and changes the symbol length of the OFDM signal when the handover detection means detects the start timing of the handover. The portable terminal according to claim 1 or 2. Timing difference measuring means for measuring a timing difference between the two OFDM signals received by the receiving means from each of two base stations at the time of the handover;
Guard time length determining means for determining the length of the guard time length to be changed by the OFDM signal control means based on the timing difference measured by the timing difference measuring means,
The OFDM signal control means, when the handover detecting means detects the start timing of the handover, changes the guard time length to a length determined by the guard time length determining means. Item 4. The mobile terminal according to any one of Items 1 to 3. Timing difference measuring means for measuring a timing difference between the two OFDM signals received by the receiving means from each of two base stations at the time of the handover;
A guard time length table that associates the timing difference with the length of the guard time length to be set for the OFDM signal;
The OFDM signal control means associates the guard time length with the timing difference measured by the timing difference measuring means in the guard time length table when the handover detecting means detects the start timing of the handover. The mobile terminal according to claim 4, wherein the mobile terminal is changed to a specified length. The receiving means receives guard time length information indicating the length of the guard time length at the time of the handover from the base station,
The said OFDM signal control means changes the said guard time length to the length shown by the said guard time length information which the said receiving means received, The Claim 1 characterized by the above-mentioned. Mobile device.   7. The method according to claim 1, further comprising a spreading factor determining unit that determines a spreading factor of the OFDM signal based on the length of the guard time length after the OFDM signal control unit has changed. A portable terminal according to claim 1. A communication system in which a plurality of base stations communicate with a mobile terminal using an OFDM (Orthogonal Frequency Division Multiplexing) signal,
The portable terminal is
Communication means for transmitting and receiving the base station and the OFDM signal;
A handover detecting means for detecting a handover start timing;
OFDM signal control means for changing a guard time length of the OFDM signal when the handover detection means detects the start timing of the handover,
The base station sets the guard time length to the same length as the guard time length changed by the OFDM signal control means at the same time that the OFDM signal control means of the mobile terminal changes the guard time length. A communication system characterized by the above. A communication method for communicating with a base station using an OFDM (Orthogonal Frequency Division Multiplexing) signal,
A communication step of transmitting / receiving the OFDM signal to / from the base station;
A handover detection step for detecting a handover start timing;
An OFDM signal control step for changing a guard time length of the OFDM signal when the start timing of the handover is detected in the handover detection step.

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